Help with unknown wall type and unknown paint type

Hi there. I am fixing up a closet in my "old house".

I have a closet that has masonry or drywall walls (thinking it is drywall). I applied Citrustrip gel (Citristrip Safer Paint & Varnish Stripping Gel - Qt. - QCG73801T at The Home Depot) I bought from Home depot and used up an entire quart. After much scraping, there is still a yellow residue from underneath the paint. I think it might be another layer of enamel paint but I am unsure. Also, there is cracking of the wall and a hole as well.

I have a large container of "Sheetrock all purpose joint compound" which I was intending to use to fill in the hole and cracks. Since I am unsure as to whether it is indeed drywall/sheetrock I could use some assistance.